This invention relates to the use of a ferritic chromium-molybdenum steel having a high resistance to the corrosion by concentrated sulfuric acid.
Sulfuric acid is usually produced by a catalytic reaction of the SO.sub.2 content of gases to form SO.sub.3 and--if the gases are dry--a subsequent absorption of the resulting SO.sub.3 in concentrated sulfuric acid or--if the gases are moist--a subsequent condensation of the resulting sulfuric acid.
In such processes, dryers, absorbers, heat exchangers, receivers associated with pumps, piping etc. are contacted with concentrated sulfuric acid having a concentration of or above 94% by weight. That sulfuric acid is an extremely aggressive fluid, which tends to subject the structural parts employed to a rapid and strong corrosion. For this reason all structural parts contacted by that sulfuric acid must consist of corrosion-resisting materials. Materials used for that purpose consist of special alloy steels, cast iron, plastics, ceramics, glass, graphite or of linings made of such materials. But the non-metallic materials have only a low mechanical strength and their processing is difficult for numerous applications. Whereas the metallic materials have a high mechanical strength, their resistance to corrosion is not sufficient in some cases or the material can be deformed only with difficulty or the material is highly expensive.
DE-C 21 54 126 discloses the use of an austenitic nickel alloy, which contains chromium, molybdenum, cobalt, manganese, copper and silicon for acids in a concentration of and above 65%. Because that alloy can be deformed only with difficulty, it can be used only for shafts, bearings, pumps, valve parts and the like elements.
DE-A-33 20 527 discloses the use of austenitic steels which contain 4.6 to 5.8% silicon. But it is difficult to process and make that material.
EP-B-0 130 967 describes four materials for use in sulfuric acids of 98 to 101% and at a temperature in excess of 120.degree. C. The highest resistance to corrosion has the ferritic material Alloy 26-1 (Material No. 1.4131, XlCrMo261), which contains up to 0.5% nickel. But that material can be processed only with difficulty and its resistance to corrosion strongly decreases as the concentration of the sulfuric acid decreases.
In EP-A-0 181 313 the ferritic material 29-4-2 has been mentioned as the second-best material, after the Alloy 26-1. for use in sulfuric acid of 98-101%. That material contains 28-30% Cr, 3.50-4.20% Mo and 2.00-2.50% Ni. That material can also be processed only with difficulty and its resistance to corrosion strongly decreases as the concentration of sulfuric acid decreases.
EP-B-0 200 862 discloses the use of a molybdenum-free alloy, which contains chromium, for use in sulfuric acid above 96% and at temperatures up to 350.degree. C. It is not significant whether the alloy has a ferritic or a ferritic-austenitic or austenitic structure. Particularly if the material has an austenitic or austenitic-ferritic structure and is used in sulfuric acids having a lower concentration that material does not have an adequate resistance to corrosion.